Desulfurization of light petroleum hydrocarbons



Aug l, 1950 w. A. HoRNE Erm. 2,516,875

DESULFURIZATION 0F LIGHT PETROL-EU HYDROCRBONS Filed Sept. 27. '1946 A ZUUOMwn-)I n DIMIE No Imwmn- NB d RG oN H Hu J h .J Ivor@ uwm 1u MF NH Nh 0 ru|mu0 M W 0H HH zOZk-uzuuumm wu Dun .rogrZOO -On. m .nw www m woman 04o mo 3305 O Patented Aug. l, 1950 DESULFURIZATION F LIGHT PETROLEUM HYDROCARBONS William A. Horne, Oakmont, and James F. J ung, Pittsburgh, Pa., assignors to Gulf Research & Development Company, Pittsburgh, Pa., a corporation of Delaware Application september 27, 194s, serial No. 699,671

(o1. 19e-2s) 13 Claims. l

This invention relates to the desulfurization of light petroleum hydrocarbons and more particularly to a method of contacting light petroleum hydrocarbons with a contact agent to remove sulfur therefrom.

Gasolinefand other light cuts obtained from high sulfur crudes such as those from the Permian Basinof West Texas have long been a problem of utmost importance to the renner. Besides the diiiiculties involved in handling, these hydrocarbons have undesirable properties such that they are only useful after removal of most of the sulfur.

The dimculties are particularly pronounced with sour cracked gasolines which are more difcult to desulfurize than straight run gasolines. The most practiced commercial process is known as acid treating and it has the distinct disadvantage of not only lowering the quality of the product as judged by octane number but it also involves considerable loss of product.

Mercaptans in gasoline may be converted into disuliides by the process known as doctor sweetening" and other similar processes. However, the disuldes are not removed but largely remain dissolved in the gasoline resulting in only slight sulfur reduction and generally in a lowering of octane number.

Newer solvent methods such as the solutizer and similar processes actually remove mercaptans from gasoline which results in an increase in lead susceptibility, as well as the desired sweetening. These processes do not remove other type sulfur compounds which are allowed to remain in the product.

An object of the present invention is to remove sulfur from light petroleum hydrocarbons with no substantial reduction in cleiin content.

A further object of the present invention is to remove sulfur in any form including hydrogen sulfide from light petroleum hydrocarbons with no substantial reduction in olefin content.

These and other objects are achieved by the process of the present invention in which a li eht petroleum hydrocarbon such as straight run or cracked gasoline or naphtha is passed, together with hydrogen, over a contact agent comprising nickel oxide supported on activated alumina. Temperatures in the range 600 to 800 F. are used.

The process generally comprises mixing the charge with hydrogen and passing the mixture over a contact agent consisting initially of nickel oxide supported on activated alumina. During the reaction the nickel oxide is converted into nickel sulde and the reaction is discontinuedl when so much nickel sulflde is present that `the end product contains substantial hydrogen sulilde.

Hydrogen is present in the range of from 100 to 1000 cubic feet per barrel of liquid charge. Pressures used are in the range of from 100 to 500 pounds per square inch gauge; a space velocity of from 1.0 and 6.0 volumes of liquid charge per hour per volume of contact agent. It will be understood that the preferred operating conditions Ior the process necessarily vary de ending upon the sulfur content and olen centen of the A charge stock but the above conditions generally apply to light petroleum charge stocks. It is undesirable to have hydrogen sulfide in the end product and for this reason when about 50 to 60 per cent oi' the original nickel oxide becom'es nickel sulilde, the contact agent should be regenerated. The regeneration is accomplished by burning the contact agent in the presence of an oxygen containing gas to reconvert it to nickel oxide and to burn oii the carbonaceous deposit.

Our process begins with nickel oxide and during the course of reaction this is converted to nickel sulde thereby removing the sulfur compounds from the charge stock and liberating the oxygen of the nickel oxide as water. There may also be simultaneous reduction of some of the nickel oxide to lower nickel oxides, such as N or metallic nickel with concurrent formation of water. Subsequently, part of these lower oxides and/or metallic nickel probably reacts with the sulfur compounds in the charge stock. It is pos sible that during the reaction the nickel oxide, nickel suliide, or nickel metal may act as catalysts, but the net result is that of the reaction between nickel oxide and the sulfur. Apparently the use oi nickel oxide as the starting material inhibits the cracking activity of the subsequently produced metallic nickel to such an extentthat cracking to gas and coke is not excessive. Furthermore, since the total time spent by any of the contact in the metallic state is limited, rapid loss of activity through sintering is avoided as well as decrease in unsaturation. The maximum processing period is limited to the interval during which no substantial amount of hydrogen sulfide is liberated from the reactor.

The presence of hydrogen sulfide in the effluent product indicates that essentially all of the readily available nickel oxide and nickel has been converted to nickel sulfide. At this time it is necessary to regenerate the contact with an oxygen-containing gas which converts the nickel suliide back to nickel oxide, and simultaneously generation as nickel sulfa. However, this nickel I sulfate does not accumulate during subsequent cycles under our preferred conditions and apparently is not disadvantageous. The herein proposed theory oi' the action of the contact agent should not be interpreted to limit the claims in any manner.

A flow sheet of the equipment `for carrying out our desuliurization process is shown in the attached drawing in which the charge stock enters line l and is pumped by means of pump 2 through line 3, product heat exchanger t and line 5 into heater E where it is heated to reaction temperature either before or after mixing with the hydrogen or hydrogen-rich gas from line 29. The mixture of charge and hydrogen or hydrogen-rich gas at the desired reaction temperature passes through line 1, valve 8 and line 0 into the reaction chamber le containing the preferred contact.

leased through valve 23 and line 20. The sep-l arated gases contain hydrogen which may be recovered for recycle if desired in any conventional manner such as an oil absorption and stripping i system. The hydrogen-rich gas from the high pressure separator may also require some hydrogen enrichment which can also be carried out in a conventional manner. These gases normally pass through line 25, are compressed by pump 26, pass through line 2l and admix with fresh or make-up hydrogen from line 28 and valve 39. This hydrogen-rich gas is preheated in heater 6 either separately or in admixture with the charge stock. This mixture then passes through line l as described previously.

During this on-stream period valves 30, 3| and 32 remained closed. At the completion of the on-stream period which varies depending on the charge stock and the conditions of reaction, pumps 2 and 26 are turned oil and valves 8, l 2 and 36 are closed. The pressure is released on contact bed I0 by means of valveV 3i and the reactor is vacuum purged in conventional manner. Alternatively valve 3I and valve 32 may both be opened and the contact purged with an inert gas or steam. The purpose of the purge is to recover valuable hydrocarbons which remain in the contact bed. Following the purge, valve 3i is closed and valves 30 and 32 are opened and an oxygen containing gas is admitted to the reactor by means of line 3l, valve 30 and line 9. The admission of this oxygen containing gas in controlled amounts regenerates the contact converting it to nickel oxide and simultaneously forms sulfur dioxide which is vented from the reaction chamber by means of line Ii, valve 32 and line 35 together with combustion gases from the carbo"- naceous deposit. This regeneration oir-gas contains recoverable quantities of sulfur dioxide in amounts depending upon the quantity of sulfur originally present as nickel sulfide as well as the oxygen content of the regeneration gas. This sulfur dioxide may be recovered in any normal manner such as by solvent absorption and stripping. The sulfur dioxide-free regeneration oigas may then serve to dilute the fresh regeneration gas admitted to the reaction chamber. Alternatively the sulfur dioxide-containing regeneration gas may serve to dilute the fresh re" generation gas. After regeneration is complete, inert gas or steam may again be used to flush the system by closing valve 30 and opening valve 3i or the reaction chamber may be vacuum purged. The -unit is then put on-stream again by closing valves 3i and 32, opening valves 8, l2 and 36 and starting pumps 2 and 26.

The foregoing description of the apparatus employed has been used as an example only. Mu..- tiple or single beds of iixed contact or fluidized contact as well as systems in which the contact is continuously charged and removed from the reaction zone followed by external regeneration are not excluded.

The preferred contact agent was prepared as follows: Dried granular 3 to 6 mesh activated alumina was impregnated with an aqueous nickel nitrate solution followed by drying at 260 F. and subsequent calcining at 800 F. to decompose the absorbed nickel nitrate to nickel oxide. This sequence of impregnation, drying and calcining was twice repeated so that the iinal product contained 29.3 per cent nickel oxide and 70.7 per cent alumina.

The charge was a West Texas cracked gasoline distillate having a gravity of 54.9 API, a. bromine number of 46.4, a specic gravity at /60 F. of 0.7591 and a sulfur weight per cent of 0.24. Following are the results obtained using the conditions indicated:

Hydrogen Liquid Reoovery Product Space Vel. Through- Aver. cycle No. gw; im' voi/v51. put, React. Lam

-g xm. v51/V61. TF. Charge Consumed waper v51. per S Wg s G Br. Cart/B51. cu. F./Bb1. 65111; cent P- f N6.

per cent4 1. 250 4. 04 24. s 615 481 v5 98.42 9s. 61 0.100 0.1511 49. 2 250 5. 23. 4 707 505 75 95. 95 96. 20 0. 095 0. 7571 50. 5 250 4. 2l 25. 3 703 232 54 97. 89 98. 14 0. 105 0. 7589 51. 0 250 4. 05 24. 3 731 975 121 96. 08 96. 33 0. 0439 0. 7571 42. 2 4. 10 24. 6 708 482 99 98. 28 98. 53 0. 129 0. 7599 51. 8 250 4. 00 24. 0 806 494 74 96. 29 96. 54 0. 105 0. 7587 45. l 250 3. 63 2l. 8 725 570 70 98. 49 98. 74 0. 106 0. 7587 48. 7 500 3. 91 23. 5 728 504 136 95.09 95. 33 0. 0397 0.7571 36.1 250 4. 14 24. 85 807 478 95 95. 93 96. 18 0. 114 0. 7591 44. 5 250 4. 00 48.0 708 494 76 98. 81 99. 01 0. 112 0. 7609 47. 4 250 4.00 24.0 616 494 12a 98.61 99.86 0.101 0.1616 46.0 l2. 250 2. 00 24.0 719 494 84 98. 30 98. 55 0. 104 0. 7603 54. 6 250 5.93 21.4 795 555 91 95.90 90.15 0.126 0. 760s 45.2

This invention is concernedwith 'low boiling petroleum hydrocarbons which for. purposes of this application maybe deilnedas those'normally liquid petroleum fractions having an ASTM end point up to 600?. This includes straight run or cracked gasoline and naphtha.-"Thc process, however, is particularly suitable to treating cracked sulfur containing gasoline. 1

While the method oi' preparation of the contact agent has `been described as impregnation, other methods of preparation may be used such as preciptation, decomposition or other vnickel salts, etc. However it is essential that Alow temperatures beused since high calcining temperatures deactivate the alumina.

As described above the preferred operating conditions for the process necessarily vary" within certain ranges depending upon the charge stock. In general the conditions are as follows:

Hydrogenconcentration, cu. it./bbl- 100-1000 At temperatures below 600 F.the desuliurizing activity of the contact diminishes whereas above 800 F. excessive cracking reactions result in decreased product recovery and rapid coke formation which deactivates the contact; At pressures below 100 p. s. i. g.the partial pressure oi hydrogen is not suillcient to maintain desuliurizing activity nor to suppress cracking reactions which result in coke formation. Increasing the pressure above 500 p. s. i. g. results in only a slight incremental gain in desuliurization and a decrease in bromine number and is thus not commercially desirable.

Space velocities below 1 0-vol." of charge per hour per volume of contact agent result in excessive cracking and olen saturation reactions Acaused by the long contact time and at space velocities above 6.0 the contact time is too short to provide suilcient desulfurization-` Hydrogen concentrations below, 100 cu. ft. bbl. of charge stock are not suicient to maintain desuliurization activity nor to suppress cracking reactions properly. Hydrogen concentrations above' 1000 cu. it. per bbl. result in only a slight incremental gain in desulfurization and cause a disadvantageous decrease in bromine number. Higher space velocities and hydrogen concentrations than 6.0 vol. charge per hour per volumev .contact and 1000 cu. ft. per barrel, respectively, result in insufficient contact time to allow complete reaction of the hydrogen sulfide and it is evolved from the reactor within a relatively short processing period. The volumes of charge stock passed over one volume of contact during the processing period depends on the per cent sulfur :'n the charge stock and the per cent nickel oxide in the contact but normally is in the range of l to 100. This throughput volume is limited by tho period s any accumulated coke deposit.

`Ihe regeneration vpressure is'adjusied within the indicated range` 'to-obtain the' optimum regeneration time which -ent gases may include steam, iiue gas, regeneraduring which no hydrogen sulfide is evolved from the contact bed.

The optimum regeneration conditions are approximately:

Temperature, "F 10000-1300 Pressure, p. s. i. g 0-500 Diluent gas to air ratio, vol/vol 0-20 tion o-gas, etc.

' The present process has many advantages.

4Sulfur content of the gasoline is reduced and `the-bromine number remains .essentially unchanged.l Clear and leaded octane numbers by both motor and research methods are increased and lead susceptibility is improved. According to the present process doctor sweet gasoline is a produced in one step instead of the three stepsoi acid treating, re-running, and sweetening. In addition, slud'ge, polymerization and some handling losses are eliminated. As compared to solvent processes the present invention has the advantage that the sulfur content of the product is lower and octane numbers are higher. Further, the present process is more ilexible as to the charge since naphthas of higher boiling range may be desulfurized. Charge stocks of low mercaptan content which are essentially unchanged by the solvent processes are desuliurized by our process.

These advantages accrue by the use ot our process because the sulfur content is reduced and the degree of unsaturation of the product remains essentially the same as the charge. This latter point in particular serves to distinguish our process from other processes employing hydrogen. It is attributable to the use of our specific contact agent and operating conditions which yield very speciilc desulfurizing action and very little hydrogenation of oleilns.

The process is not to be confused with hydrocenation processes Yusing metallic nickel as a catalyst. We have found that ii' a nickel oxide is reduced with hydrogen prior to charging the `light petroleum oil, the efficiency of desulfurization is markedly reduced due to the high cracking activity oi `reduced nickel which results in excessive gas yields and rapid formation of a coke deposit on the contact.

Y `In the following claims when we speak of absorbing sulfur from the hydrocarbon vapors it is to be understood to'mean that the sulfur contained in the vapors, in whatever form that it is present therein, is taken up by the reaetant vdue to the formation of nickel sulfide.

What we claim isz 1. A process for desulfurizing low boilingr p".- troleum hydrocarbons without substantial change in oleiirr content comprising passing the vapors thereof over a contact agent consisting initially of unreduced nickel oxide supported on activated alumina at a temperature in the range of from 600 to 800 F. and at a pressure in the range 100 to 500 'pounds per square inch gauge, and in tbe presence'of hydrogen, absorbing sulfur from the vapors to form nickel sulfide, terminating the passage of vapors before a substantial amount of hydrogen suliide appears in the eiiluent and before about 60 per cent of the available nickel content of the contact lhas been converted into nickel sulfide, regenerating the contact by converting the nickel sulfide thus formed into nickel oxide and re-using the regenerated contact in the process. y

2. A process for desulfurizing low boiling petroleum hydrocarbons without substantial change in olefin content comprising passing the vapors andere j thereof over a contact agent consisting initially of unreduced nickel oxide supported on activated aluminaat a temperature in the range of from 600 to 800 F. at a pressure above about 100 pounds per square inch gauge at a space velocity in the range 1.0 to 6.0 liquid volumes of charge per hour per volume of contact agent, and in the presence of hydrogen, absorbing sulfur from the vapors to form nickel sulfide, terminating the passage of vapors before a substantial amount of hydrogen sulfide appears in the euent and before about 60 per cent of the available nickel content of the contact has been converted into nickel sulfide, regenerating the contact by converting the nickel sulfide thus formed into nickel oxide and re-using the regenerated contact in the process.

3. A process for desulfurizing low boiling petroleum hydrocarbons without substantial change in olefin content comprising passing the vapors thereof over a contact agent consisting initially of unreduced nickel oxide supported on activated alumina at a temperature in the range of from 600 to 800 F. at a pressure above about 100 before about 6.0 per cent of the available nickel content of the contact has been converted into nickel sulfide, regenerating the contact by converting the nickel sulfide thus formed into nickel oxide and re-using the regenerated contact in the process.

4. A process for desulfurizing low boiling petroleum hydrocarbons Without substantial change in olefin content comprising passing the vapors thereof over a contact agent consisting initially of unreduced nickel oxide supported on activated alumina at a temperature in the range of from 600 to 800 F. and at a pressure in the range 100 to 500 pounds per square inch, at a space velocity in the range 1.0 to 6.0 liquid volumes of charge per hour per volume of contact agent and in the presence of hydrogen, said hydrogen being present in a concentration range of from 100 to 1000 cubic feet per barrel of charge, absorbing sulfur from the vapors to form nickel sulfide, terminating the passage of vapors before a substantial amount of hydrogen sulfide appears in the effluent and before about 60 per cent of the available nickel content of the contact has been converted into nickel sulfide, regenerating the contact by converting the nickel sulfide thus formed into nickel oxide and re-using the regenerated contact in the process.

5. lA process for desulfurizing straight run gasoline comprising passing the vapors thereof over a contact agent consisting initially of unreduced nickel oxide supported on activated alumina at a temperature in the range of from 600 to 800 F. and at a pressure in the range 100 to 500 pounds per square inch gauge, at a space velocity in the range 1.0 to 6.0 liquid volumes of charge per hour per volume of Contact agent and in the presence of hydrogen, said hydrogen being present in a concentration range of from 100 to 1000 cubic feet per barrel of charge, absorbing sulfur from the vapors to form nickel sulfide, terminating the passage of vapors before a substantial amount of hydrogen sulfide appears in the efiiuent and before about 60 per cent of the available nickel content of the contact has been converted into nickel sulfide, regenerating the contact by converting the nickel sulfide thus formed into nickel oxide and re-using the regenerated contact in the process.

6. A process for desulfurizing cracked gasoline without substantial change in olefin content comprising passing the vapors thereof over a contact agent consisting initially of unreduced nickel oxide supported on activated alumina at a temperature in the range of from 600 to 800 F. and at a pressure in the range to 500 pounds per square inch gauge, at a space velocity in the range 1.0 to 6.0 liquid volumes of charge per hour per volume or" contact agent and in the presence of hydrogen, said hydrogen being present in a concentration range of from 100 to 1000 cubic feet per barrel of charge, absorbing sulfur from the vapors to form nickel sulfide, terminating the passage of vapors before a substantial amount of hydrogen sulfide appears in the effluent and before about 60 per cent of the available nickel content of the contact has been converted into nickel sulfide, regenerating the contact by converting the nickel sulfide thus formed into nickel oxide and re-using the regenerated contact in the process. r

7. A process for desulfurizng petroleum naphtha without substantial change in olefin content comprising passing the vapors thereof over a contact agent consisting initially of unreduced nickel oxide supported on activated alumina at a temperature in the range of from 600 to 800 F. and at a pressure in the range 100 to 500 pounds per square inch gauge, at a space velocity in the range 1.0 to 6.0 liquid volumes of charge per hour per volume of contact agent and in the presence of hydrogen, said hydrogen being present in a concentration range of from 100 to 1000 cubicfeet per barrel of charge, absorbing sulfur from the vapors to form nickel sulfide, terminating the passage of vapors before a substantial amount of hydrogen sulfide appears in the effluent and before about 60 per cenl of the available nickel content of the contact has been converted into nickel sulfide, regenerating the contact by converting the nickel sulfide thus formed into nickel oxide and re-using the regenerated contact in the process.

8. A process for desulfurizing low boiling petroleum hydrocarbons vvithout substantial change iny olefin content comprising passing the vapors thereof over a contact agent consisting initially of unreduced nickel oxide supported on activated alumina at ay temperature in the range of 600 to 800 F. at a pressure above about 100 pounds per square inch gauge and in the presence of hydrogen, absorbing sulfur from the vapors to form nickel sulfide, terminating the passage of vapors before a substantial amount of hydrogen sulfide appears in the effluent and before about 60 per cent of the nickel content of the contact has been converted into nickel sulfide, regenerating the contact by converting the nickel sulfide thus formed into nickel oxide and re-/using the regenerated contact in the process.

9. A process for desulfurizing low boiling petroleum hydrocarbons without substantial change in olefin content comprising passing the vapors thereof over a contact agent consisting initially of unreduced nickel oxide reactant supported on activated alumina at a temperature in the range of 600 to 800 F. at a pressure above about 100 pounds per square inch gauge and in the presence of hydrogen, absorbing sulfur from the vapors to form nickel sulilde, terminating the passage of vapors when the reactant contains about 50 per cent of nickel sulfide, regenerating the contact by converting the nickel sulfide thus formed into nickel oxide and re-using the regenerated contact in the process.

10. A process for desuliurizing low boiling petroleum hydrocarbons without substantial change in olefin content comprising passing the vapors thereof over a contact agent consisting initially of unreduced nickel oxide reactant supported on activated alumina at a temperature in the range of 600 to 800 F. at a pressure above about 100 pounds per square inch gauge and in the presence of hydrogen, absorbing sulfur from the vapors to form nickel sulfide discontinuing the passage of said vapors when the reactant contains about 50 per cent of nickel sulflde, regenerating the contact agent by passing an oxygen containing gas over it and re-using the regenerated contact in the process.

1l. A process for desulfurizing low boiling petroleum hydrocarbons without substantial change in olefin content comprising passing the vapors thereof over a contact agent consisting initially of unreduced nickel oxide reactant supported on activated alumina at a temperature in the range of 600 to 800 F. at a pressure above about 100 pounds per square inch gauge and in the presence of hydrogen, absorbing sulfur from the vapors to form nickel sulfide discontinuing the passage of said vapors when the reactant contains about 50 per cent of nickel sulfide, regenerating the contact agent by passing an oxygen-containing inert gas over it at a temperature in the range 1000" to 1300* F. and pressure o1 from atmospheric to 500 pounds per square inch and re-using the regenerated contact in the process.

12. A process for desulfurizing low boiling petroleum hydrocarbons without substantial change in oleiln content comprising passing the vapors thereof over a contact agent consisting initially o! unreduced nickel oxide reactant and activated alumina at a temperature in the range 600 to 800 F. at. a pressure above about 100 pounds per square inch gauge and in the presence of hydrogen, absorbing sulfur from the vapors to form nickel sulfide discontinuing the passage of said A cent of nickel sulilde, regenerating the contact aent by passing a diluent gas-air mixture thereover at a temperature in the range 1000 to 1300 F. and a diluent gas to air ratio of 0 to 20 and reusing the regenerated contact in the process.

13. A process for desulfurizing cracked gasoline without substantial change in olen content comprising passing the vapors thereof over a contact agent consisting initially of unreduced nickel oxide supported on activated alumina at a temperature of about 700 F., a pressure of about 250 p. s. i. g., a space velocity of about 4 volumes of charge per hour per volume of contact agent in the presence of hydrogen, said hydrogen being present in the concentration of about 500 cubic feet per barrel of charge, absorbing sulfur from the vapors to form nickel sulde, terminating the passage of vapors before a substantial amount of hydrogen sulfide appears in the eilluent and before about 60 per cent of the available nickel content of the contact has been converted into nickel sulfide, regenerating the contact by converting the nickel sulfide thus formed into nickel oxide and re-using the regenerated contact in the process.

WILLIAM A. HORNE. JAMES F. JUNGE.

REFERENCES CITED The following references are of record in the le oi this patent:

UNITED STATES PATENTS Number Name Date 1,955,297 Jennings Apr. 17, 1934 2,037,792 Ipatieil Apr. 21, 1936 2,063,113 Morrell Dec. 8. 1936 2,236,216 Lyman et ai Mar. 25, 1941 2,298,346 Carson et al. Oct. 13, 1942 2,392,579 Cole Jan. 8, 1946 2,413,312 Cole Dec. 31, 1946 2,422,372 Smith et al. June 17, 1947 2,431,920 Cole ..`Dec. 2, 1947 FOREIGN PATENTS Number Country Date 348,768 Italy May 31, 1937 

1. A PROCESS FOR DESULFURIZING LOW BOILING PETROLLEUM HYDROCARBONS WITHOUT SUBSTANTIAL CHANGE IN OLEFIN CONTENT COMPRISING PASSING THE VAPORS THEREOF OVER A CONTACT AGENT CONSISTING INITIALLY OF UNREDUCED NICKEL OXIDE SUPPORTED ON ACTIVATED ALUMINA AT A TEMPERATURE IN THE RANGE OF FROM 600* TO 800*F. AND AT A PRESSURE IN THE RANGE 100 TO 500 POUNDS PER SQUARE INCH GUAGE, AND IN THE PRESENCE OF HYDROGEN, ABSORBING SULFUR FROM THE VAPORS TO FORM NICKEL SULFIDE, TERMINATING THE PASSAGE OF VAPORS BEFORE A SUBSTANTIAL AMOUNT OF HYDROGEN SULFIDE APPEARS IN THE EFFLUENT AND BEFORE ABOUT 60 PER CENT OF THE AVAILABLE NICKEL CONTENT OF THE CONTACT HAS BEEN CONVERTED INTO NICKEL SULFIDE, REGENERATING THE CONTACT BY CONVERTING THE NICKEL SULFIDE THUS FORMED INTO NICKEL OXIDE AND RE-USING THE REGNERATED CONTACT IN THE PROCESS. 